[tex]\boxed{sin t=\frac{2\cdot tg \frac{t}{2}}{1+tg^2 \frac{t}{2}}}\\ \\ \\ \\ sin 2a=\frac{2\cdot tg \frac{2a}{2}}{1+tg^2 \frac{2a}{2}}}\\ \\ sin 2a=\frac{2\cdot tg a}{1+tg^2 a}\\ \\ sin 2a=\frac{2\cdot \frac{x}{7}}{1+(\frac{x}{7})^2}\\ \\ sin 2a=\frac{\frac{2x}{7}}{\frac{49}{49}+\frac{x^2}{49}}\\ \\ sin 2a=\frac{2x}{7}:\frac{49+x^2}{49}\\ \\ sin 2a=\frac{2x}{7}\cdot \frac{49}{49+x^2}\\ \\ sin 2a=\frac{2x\cdot 7}{49+x^2}=\frac{14x}{49+x^2}[/tex]
